Hydrocarbon synthesis catalysts come in a variety of types, perhaps, the most useful being supported catalysts where the catalytic metal may be a Group VIII metal, e.g., Fe, Co, Ru, and the support is any relatively inert material known in the art as a catalyst support, e.g., difficulty reducible inorganic refractory oxides or kielseguhr, diatomeaceous earths, etc. The catalytic metal can be incorporated into the support in many different ways, e.g., kneading, impregnation, spraying, etc. Nevertheless, regardless of the method of preparation, the final activating step is usually a treatment with hydrogen or a hydrogen containing gas. This final activating step, whether a first or second or subsequent hydrogen treating step, is usually performed ex situ, but can be performed in the reactor just prior to start up, particularly for fixed bed units.